Genetics, the fact that many of these guys get to know the cook well and do get more food, and the fact that many have nothing else to focus on but whatever exercises they can do. With some facilities removing weight rooms, this may be becoming more difficult. Also, "huge" is relative. It is not uncommon for a guy to come out weighing 200+lbs, but who here considers that "HUGE"? Developed? Yes. It might even be impressive compared to many who don't train, but many of these guys who are capable of doing that would be even bigger had they had better nutrition and the same focus on the outside.
Bottom line, those who desire physical change to the point that it becomes a major priority can see even more impressive change. How many people actually have that focus on the outside? I guarantee when I go to the gym this afternoon, there may be 2 people out of many who actually look like they make bodybuilding a priority in their lives.
If half of the slackers on the outside had even half of the focus many of these guys in prison have (due to lack of other stimulus), several more would be making significant progress.